The complexities of golf club design are known. The specifications for each component of the club (i.e., the club head, shaft, grip, and subcomponents thereof) directly impact the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have specific performance characteristics.
The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, center of gravity location, rotational moment of inertia, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club designers, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as the inclusion of a hosel or a shaft attachment means, perimeter weights on the club head, and fillers within the hollow club heads.
Golf club heads must also be strong to withstand the stresses that occur during repeated collisions between the golf club and the golf balls. The loading that occurs during this transient event can create a peak force of over 2,000 lbs. Thus, a major challenge is to design the club face and club body to resist permanent deformation or fracture. Conventional hollow metal wood drivers made from titanium typically have a uniform face thickness exceeding 2.5 mm or 0.10 inch to ensure structural integrity of the club head.
Players generally seek a metal wood driver and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact is dictated by the magnitude and direction of the ball's initial velocity and the ball's rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed, further influence the ball's flight. However, these environmental effects are beyond the control of the golf equipment designers. Golf ball landing accuracy is driven by a number of factors as well. Some of these factors are attributed to club head design, such as center of gravity and moment of inertia.
The current trend in golf club manufacturing is to produce large volume club heads in order to maximize the moment of inertia of the club head. Concerned that improvements to golf equipment may render the game less challenging, the United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf equipment. These performance specifications dictate the size and weight of a conforming golf ball or a conforming golf club. USGA rules limit a number of parameters for drivers. For example, the volume of drivers has been limited to 460±10 cubic centimeters. The length of the shaft, except for putters, has been capped at 48 inches. The driver club heads must fit inside a 5-inch square and the height from the sole to the crown cannot exceed 2.8 inches. The USGA has further limited the coefficient of restitution of the impact between a driver and a golf ball to 0.830.
The USGA has also observed that the rotational moment of inertia of drivers, or the Club's resistance to twisting on off-center hits, has tripled from about 1990 to 2005, which coincides with the introduction of oversize drivers. Since drivers with higher rotational moment of inertia are more forgiving on off-center hits, the USGA was concerned that further increases in the club head's inertia may reduce the challenge of the game, and instituted in 2006 a limit on the moment of inertia for drivers at 5900 g·cm2±100 g·cm2 (590 kg mm2+10 kg·mm2) or 32.259 oz·in2±0.547 oz·in2.
The USGA limits moment of inertia for drivers, as the calculated moment of inertia with respect to a vertical axis through the center of gravity of the club head. Larger MOIs about the vertical axis preserve more ball speed on off-center impacts. However, when a golf club head approaches a golf ball during the downswing the golf club head rotates around the shaft or hosel of the club. The moment of inertia around this “hosel axis” tends to be significantly larger than the moment of inertia around the vertical axis through the center of gravity. The moment of inertia about the hosel or shaft axis is the rotational mass or “foot print” of the club that the golfer must work to overcome just prior to impact in order to hit a straight shot. In large-volume drivers manufactured to have large moments of inertia around the vertical axis, this difference in moment of inertia is even more exaggerated. Players may find it difficult to control a club head having a very large moment of inertia around the hosel axis, because it requires more work during the downswing to “square” the face and hit straight shots.
Though methods of optimizing the mass properties of golf club heads exist, there remains a need in the art for a golf club head having a smaller volume or footprint, and/or an optimized moment of inertia with respect to the hosel axis or rotational footprint. Further, there remains a need in the art for a golf club head having a large moment of inertia around the vertical axis through the center of gravity and a smaller moment of inertia around the hosel axis relative to the moment of inertia about the hosel axis of large volume drivers.